SB SIT •^tMti THE POTAMOGETONS IN RELATION TO POND CULTURE ;:::::;::: By Emmeline Moore From BULLETIN OF THE BUREAU OF FLSHERIES, Volume XXXIII, 1913 Document No. 815 : : : : : .- ■ .- .• .• .■ .• .. /^^„g^ y,,/^, ^S, 191 5 WASHINGTON :::::: GOVERNltENT PRINTING OFFICE 191S Glass _c Bonk i : < •y I 0- THE POTAMOGETONS IN RELATION TO POND CULTURE :::::::::: By Emmeline Moore From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXXIII, 1913 Document No. Si 5 : : : : : : : : : : : : .- .- Issued July 28, igi^ V/7 WASHINGTON :::::: GOVERNMENT TRINTING OFFICE THE POTAMOGETONS IX RELATIOX TO POND CULTL^flE By Ejnmeline Moore Contribution from the Dc{hjrtnu-nt of Limnology, Cornell Univerriiy CONTENTS. Page. Introduction 255 Historical 255 Species of Potamogeton investigated 260 General surv-ey of life conditions of the species investigated 261 P. americanus 261 P. amplifolius 261 P. heterophyllus 262 P. perfoliatus 263 P. crispus 264 P. zosterifolius 264 P. obtusifolius 265 P. filiformis 265 P. pectinatus 265 P. Robbinsii 267 Summan,' of cultural features 268 Natural and artificial propagation ._ 268 Propagation by tubers 269 Propagation by tuberous rootstocks 272 Propagation by subterranean stems not tuberous 273 Propagation by winter buds 273 Propagation by burs 274 Propagation by fragments of stems 277 Propagation by seeds 278 Production of seeds and vegetative propagative structures 279 Economic aspects of Potaraogetons 281 Species of Potamogeton and the animals foraging upon them 284 Conclusion 285 Bibliography 286 Explanation of plates 289 253 THE POTAMOGETONS IN RELATION TO POND CULTURE. By EMMELINE MOORE, Contribution from the Department of Limnology, Cornell University. INTRODUCTION. The cultivation of lakes, ponds, and streams follows as a natural consequence the biological investigation of the aquatic life within them. Herbivores and carnivores live their life in the water, and if we ponder over their means of sustenance we are struck bv the fact that the natural food supply has rarely been augmented by cultural methods. "The larger aquatic plants," says Pond (1903), "form a hnk in the chain of nutri- tive relations that stretches from the water and soil to the higher fishes." If such is the importance of these plants, the great mass of vegetation which comes to maturity each season is a national asset. Yet the annual yield has never been estimated or given a place in the Government crop reports. Aquatic plants have contested for possession of the waters much as the grasses have contended for supremacy on land, until it may be said that the dominant forage crop of our lakes, ponds, and streams is to be found among the pondweeds, the Pota- mogetons. Variety in form, adaptability to environment, and diversity in range have all contributed their share in giving prominence to this group and in furthering a natu- ral resource whose propagation and control are vital factors in the economic relations of the life of inland waters. The object of this investigation is to present such observations and experiments on the natural and artificial propagation of the Potamogetons as will render cultural methods economical and practical. The work herein recorded was carried on at Cornell University under the direction pf Dr. James G. Needham, to whom I wish to express mv grateful thanks for help and suggestion. HISTORICAL. The cultivation of aquatic plants was an ancient occupation, one which concerned itself with the beautification of pools and fountains. In modern times, too, aquatic plants have been used in variety and profusion in the ornamentation of artificial or natural ponds. But the cultivation of aquatics from an economic standpoint is a new idea, so new, in fact, that data regarding it are just beginning to appear in bulletin form in the Government compilations of scattered and isolated experiments. In the bulletins plants of the genus Potamogeton have received the larger measure of notice because observations on the feeding habits of animals associated with them point to the important role of these plants in the economy of nature. 255 256 BULLETIN OF THE BUREAU OF FISHERIES. Further contribution to the present status of the Potamogetons incorporates of necessity a considerable body of observation pertaining to the systematic, morpho- logical, and biological aspects of this group, and renders it highly desirable to set forth the historical background of each of these three phases of the subject. John Gerarde, 1633. A beginning in the classification of the Potamogetons was made by tlie old herbalists, medical men, who found it necessary to study plants in detail in order to discriminate t]ic kinds employed for different purposes. The special virtue in Potiimogctons, for example, resided in tlie leaves, which were applied to reduce inflammation. In the herbal of John Gerarde the group Potamogeton (Ptla- inogeiton in the old spelling and pondweed or water spike in the common parlance of the time) con- sisted of four species — a broad-leaved pondweed, a narrow-leaved pondweed, a small pondweed, and a long sliarp-Icaved pondweed. There was a figure of the entire plant accomp;mied by Oie Latin and Englisli name. Then followed the "description, place, time, names, nature, and virtues agreeing with the best received opinions." A "fennel-leaved water milfoile" illustrated by a figure easily recognized as our fennel-leaved pondweed, Potamogeton pectinatus, was given a place among the MjTiophyllums. Such was one of the earliest attempts to classify the group. Chamisso and Schlechtendal, 1827. The first important monograph of the Potamogetons was the work of Chamisso and Schlechtendal, who, in Linnca, volume 2, 1827, systematized the results of scientific observation during the latter part of tlic eighteenth and the beginning of the nineteenth centuries. Under the family name of Alismacea 21 species were described and illustrated by drawings of fruit and leaf, including among tliem many of the common and widely distributed species of to-day. Several other Potamogetons were listed as uncertain in position and ditTicult to classify, a condition which holds as true to-day as then, when Chamisso and Schlechtendal struggled to bring order out of chaos in tliis puzzling group and recorded this pertinent observation: "Species Potamogetonum habitum mutantes in alias saepe transire videntur, alienaeque speciei habitum mentientcs scrutatorum irrident," which translated is, "Species of Potamogeton changing their habit seem often to pass into others, and feigning tlie habit of other species baflSe research. " Reichenb.^ch, 1845. Reichenbach's monograph of tlie Potamogetons, in his Icones Florae Gerraanicae et Helveticae, followed in 1S45. More intensive in scope tlian any preceding work, it marked a distinct advance both in the method of description and in the matter of illustration. Several reproductions, especially of flower and fruit, which were drawn with great clearness and accuracy, have found their way in the latest authoritative works on the subject. In this monograph the author introduced the figure of the so-called "biu-," the vegetative propagative body of P. crispus Linnaeus, though he apparently did not recognize its significance in the rapid propagation of this species. It is interesting to note that the figure is inserted without further description or comment. Moreover, it is erroneously drawn, and the error has been copied time without end. Irmisch, 1851. Thilo Irmisch, in a published note in Flora, 1851, first recognized the presence of tubers on P. pectinatus. Agardu, 1852. A ye;ir later J. C. Agardh, in \''erhandlungen dcr K. Schwedischen Akademie der Wissenschaften, recorded several observations on the tubers of this species of Potamogeton. Clos, D., 1856. D. Clos was the first to publish an account of the origin of the "bur" of criTptis, though his obser- vations are incomplete regarding both tlieir development and their germination. Irmisch, 1858. In a remarkable monograph by Irmisch, Uber einige Arten aus dcrnaturlichen Pflanzenfamilie der Potamogeton, the history of the development of the tuberous growths on /'. pectinatus is recorded and their morphological and anatomical structure described. The author states that, at the end of llie vcgeta- POTAMOGETONS IN RELATION TO POND CULTURE. 257 tive period in the fall, the shoots of recent formation have a singular appearance, the last two thin internodes bearing tubers at the end. At first the tuberous end resembles a conical terminal shoot or bud surrounded by scales. Internodes make their appearance and soon become thickened ; eventually the scales split and disclose a tuber of two swollen internodes. Simultaneously a slender bud forms at the distal end of the tuber, and axial outgrowths develop from the sides that bespeak the shoots of ordinary branches. These axial shoots in turn develop swollen internodes which follow two thinner ones as in the preceding case and produce a series of tubers dichasial in form. The excellent series of drawings by means of which the author depicts tlie transition from intemode to tuber leaves nothing to be desired in the morphological interpretation of them. They are clearly two modified internodes. Tuber-bearing shoots grow out of the upper leaf axils also , and follow the usual development of genera- tions of internodes with leafy shoots, besides the tuber-bearing ones in two or three series. The anatom- ical structure of the tuber resembles that of the stem excepting that all tissue not fibro-vascular is filled with starch. The observations on P. obtusifolius are incomplete, but the presence of winter buds is noted. For P. nutans and P. lucens the morphology of the rootstock, stem, and shoot is completely determined, and the details are clearly shown in the drawings. The method of branching is fundamen- tally the same in the two species. In brief, the growing tips of the rootstocks branch dichotomously, giving an erect a.xis and a horizontal one. Each generation of the developing rootstock brings forth two horizontal internodes and a bud which is the incipient erect axis. The terminal bud at the end of the horizontal axis reproduces this condition as long as the plant lives. In the development of the erect shoot, the scales, usually three in number, grade into stipular sheath, phyllodes, and foliage leaves. A two-fifths arrangement of leaves is noted and the shoots follow the same order. The winter condition of P. liicens consists of rootstocks by means of which the plant propagates itself rapidly in the spring. The internodes of these rootstocks are shorter and thicker than the ordinarj- ones and are borne in a suc- cession of three or more with terminal and axillary buds containing the incipient axes of the horizontal and erect shoots. Irmisch made observations also on P. crispus, investigating especially the "burs" or propagative shoots, although this work was anticipated in part by D. Clos in his Mode de Propagation particulier au Poiamogeion crispus L. Irmisch, however, found two forms of the hur oi crispus, the slender spicular bur as well as the stout, homy, denticulate one observed by Clos. The former bur he observed growing in the axils of detached shoots in late autumn and aftenvards breaking away from the axils and settling in tlie mud. The origin of the latter form he did not observe, but he found it in the muddy bottoms of ponds in great abundance. These " burs" or modified twigs, as Irmisch sometimes called them, he con- sidered important examples of propagative structures. In connection with these plants, Irmisch first pointed out the "Scheiden-Schiippchen, squamulas intravaginales, " scale-like structures developed at the leaf bases, having as a possible function the pro- duction of slime or mucilage for the protection of young and slender shoots. This monograph is of special importance in presenting the morphological data of a few species of Potamogeton. From time to time further contributions have been made to the subject by other investi- gators in the field, but this still remains the greatest work of its kind. As a result of these studies on the tubers of P. pectinatus, the rootstocks of P. lucens, and the burs of P. crispus, Irmisch came to appreciate the advantage of artificial propagation in this group and remarked in conclusion, in an observation that is prophetic of present day interest, " That many of the Potamoge- tons, as well as other aquatic plants, possess in a singular way that possibility of domestication which has given us the tame animals from the wild ones." ROBBINS, 1867. Thus far the work of the Potamogetons was confined principally to European species. In 1867, however, the American species were reduced to something like a complete intelligible systematic shape by Dr. G. W. Robbins, whose descriptions, as far as they came within the range, were incorporated in Gray's Manual, edition 5. Later descriptions of the western species were published as they became known. MOROXG, 1893. The greatest contribution to the literature on the North American species of Potamogeton is by Thomas Morong in his Naiadaceas of North America, a monograph which includes 37 North American species, 14 of which are confined to this country. Many of these species were studied through succes- 86309°— 1.5 2 255 25^ BILLETIX OK THE BVRR.\V OF FISHERIES. saw sr;«sftns v^" the ye«r atiid a c\\nsii1er!\l>Ie Kxiy of knowledge penaininj; to thf de\xlopnHMit oi the )^l;U1t$ WHS avvuniuUted. It is rfcwvled that 17 i>l" the dcscriKil spevies are p(v>pd$:ated >-efetatJvrtv bv one or niotv of the MK^winj: stnictxites: Rix^tstxx'kji, tubers, winter buds, and stecns. S-M-VACKAf, 1S04. The «\»rJi of Sau\-5^e.^u is jv»rtiouldrty a contribution u> the biok>s,v of the iy>t3unc^tons. While there are iKlditions in nx>ri\lK\K'*^- iind anatomy extending the ohafrv-ativwsof Innisch to other members of the senus, the UKvst iK»ie»\\rthy inwsti^ations pertain tvi the ori^n and the de\-elopment of those \-esetati\x strxtctures which greatly facilitate the multiplication of species during the \-ej:euiive perivxl. S»uvaseau dewtes a s{iecial nienMr to P. crisfrnt. He observed both fxtrmscri the so-called "btirs"' ^^f this sjxx-ics. the slender s(4cular one and the more common denticulate one. noting their ivij^in, j:t\»wth, and jenninativxn. Kxperin>ents ov>nviuctv.' ri.^ show that detached fragnvents i.\f stems v%f various fomts as P. imceans of pcopasi>tion ln^-estip^t>^.^n iV" the ,ctv>w th habit of P. •tjJuiij'- disvKxscs a condition in n>ari:ed cv\ntr»st to the abo\-e-meniioned sjvcies- No special pivpaja- tiN"e bodies exist, but the sj^ecies i>erpetuates it*lf by the continxjance v^ the tlua.>roe anchored in the muvl. a rhiivvnie which maintains itself th^^^gh the winter ivst period with the submersed ij^x>ts in \-srious st-xges v>f gtvwth, Kx^x-ri:\H-nts on seevl jerminaiivin indicate a latent perivxl lAf cvtvsivlerable vatiahiUty . In P. crisfrnt gemiinat*.>n iwurs within a w.ir; in .'"". «jja«.^ in ftwni thtv* to four ycirs. Kry»s«. too^ The tirst twv last.^tiTK-nts oj a tine <}u*rtv> wvwk. The T , ,^ns v>f the z\nush Is-cs, ^> .Mired Fr\"er, aj'ijx'arevl in icvx^ The nvM>k>^:raph incluvk-s the v. s and states .-ss w-eil as the n^c^■<5- nited sjxvies, with ac\- ; pUtes. by the artist. Rolvrt Xkv^an, who has lejvxviuced the plants in cv>Jor with sangxiiar . -i accuracy, VnKvtunately kv scieiK>e. the auth^vs death cvcurred beiore ih»s imjxxrtant wwfc was tinished, Ft>'ef havi an intimate acquaintance with the I\>tainog««iMis and their habits. He grvw many speo- meits in tanks in his garvlen, watching dewkvpcnents there and in their native haunts at different times tanx>5etv>ns in order better to classify them, K>r he rewyniied the necessity at hivin< a V.\ni: s*oes <.y specimens of the Skune form. "One set." as he says. " woold cvotain a series of k- -.o iifV^,*" - ..^ejsp. This wvutd i^'>w the way in which l»v> quite d- OS r!\>ni vMu .:; a msssnj link " -Ks a result of thesw ohsercatioas a Vm^ ami v.^ .s v>f cvxuniuatcatKuts >.\a the jxnos. undcf the title "Nc>tes oc Rvid weeds." appeMevI in •. . . .^ v>f K^anv fnvu iSS_; to iS>.v. Bkxxbtt, jSSD-roi4. In the ' rv^MT.y Mr. .\rthur 'rVr.netts "' X^^tes vm: TV>iK!weeds"' have appeared OKralartv fiv>m tSSc ^■ , . . -■'• tame. He has J>ecv>n3e the acfcoow^oiged authivity on the classif.catx-c ,.v the Jb a Oiintribotikin to the BJok-wr*" of the Oieal Lakes. Mr. A. J. Pieters ac^tes the dismbotioD of a.- ■ .-.S in diverse situalioBS. presents details of strxsctuie. aad rt-. vtacti^-m. Tbe I\>tanx«setons. be observes. Kirm a cvxtspic. »x ;.>re\iominatii^ . as a ra'.e . in aquatic associatioits or dourishii:; ia is. . sa\-:i^ exempKSes the latter cooeKlion in that it thrives in a son- t«atett sandhar, whe»e its ranoers ramify in all directso>os aston^ the stooes aad pebbles, and its ivv»cs }V-,- — ■ •'-. "-■' — ; •■ 'V- -.its erf swoctute which ate %aKd K« P. *»mk-««» <- ; : •-• The s:x ^e^^ iona. whc»« leaves are sabmetyed. k* withssa.- -.a- The A^ . vro*c«U.oc wi«l«rb«ads^ lejjfeser.; -..-c CTOie .x-c oi— ^ - • ■ •>- ■ ■ - -~cal Ktetanae of aquatic pUats haw bee- — ■ -- ^v R H. IXaad. Twv'f^ -t. Ia the firs*. The Pki»,yical Reiatjoc r«aatstothe rOTAMOGETONS IN KULATION TO IMND CULTURE. 259 Substratum, tlio .uiUior showed that nvitcd aquatics depoiul on the soil substratum for the siiiinly of nitrates. In <.xmducliiig the exporimcnts \Mriotis aqu.itio phuits wore used, amou;; which were /'. JHr- foUatus and T. oHusiJoUus. It w.\s found th.it both of these pl.uits are deiK-ndent on the soil substratum for optimum growth, though the cuttings which were employed behaved differently in manner of growth: P. petfolkitus sliowcd ;ui iucre;ise of growth through the development of new rhizomes; P. oblnsifolius manifested it in a continuation of tlie brandies already present. The behavior of P. pcr/olialus is in accord with the observations of S;iuvagcau in his experiments on the propagation of Potamogotous by iragnieuts of stems. The second pajH-r of the antlior. The I.;uT;er Aquatic X'egctation. to appear inW^u'd's .-Vmerican Fresh-water Biology, supplements the work of tlie lirst by additional observations, discussions, juid generalizations. Kioin his observations on the substratum of the hu^er aquatics it appears that tliey may be found growing on gravelly, siuidy, or loamy soil, tlie loamy soil supporting the greatest variety of species. Direct experiments on this point, witli die natunU conditions reproduced as nearly as possible, bciU'out thisobserA-ation. The author states, moreover, that the cluir.teter of the soil is so iniportiuit a factor that it is possible to predict the nature of the Iwttom from the species that are found gniwing in it. For example, "Among the ishuids of western I^jike l{rie Polus is an exception, since it was collected from spring pools at all times of the year. These studies ha\e afforded an opportunity to observe the animals that are inti- mately associated with the Potamogetons. Such have been noted, especially those forms which depend upon these plants for food, support, or shelter. POTAMOGETONS IN RELATION TO POND CULTURE. 261 GENERAL SURVEY OF LIFE CONDITIONS OF THE SPECIES INVESTIGATED. POTAMOGETON AMERICANUS. This species, which has been grown from seed and cultivated through two succes- sive seasons, will receive more specific treatment later under the caption "Natural and artificial propagation." In its natural habitat this plant has been observed growing near the mouth of Fall Creek, a tributary of Lake Cayuga, and in a near-by cove of the lake, at varying depths of 3 to 4 feet. It has been observed also at Spencer Lake, at about the same depth but in much swifter water. In the latter situation the blades of the leaves are conspicuously attenuated. According to Fryer (1900), who has observed this plant in various localities, it is a plant of upland streams and rivers rather than of stagnant waters. B}- uprooting the entire plant in the growing season, it is found that the stem springs from a rootstock that is deeply anchored in the mud, where new shoots radiate horizontally from the established parts of the plant. During the summer these young rootstocks produce large buds at their tips (fig. 6). After the plant dies down, which may occur as early as August, the subterranean system remains intact for several weeks. The new rootstocks, however, carrying the buds at the tips, become eventually detached through the disorganization of the parent stem and in time die away, leaving but little beyond the isolated buds to perpetuate the plant the following spring. Such buds, since they remain in a quiescent state during the winter, may be called winter buds or hibemacula, a term applied to structures of a similar nature and function. Mr. A. J. Pieters (1901) doubtless referred to propagative structures of this kind when he recorded for P. atnericanus (P. loin:hitcs) "extensive runners bearing buds at their ends," though no figures are given and no further observations are noted. Fryer (1888) mentions an autumnal state of P. americanus {P. fluitans) in which the leaves are all narrowly linear or grasslike. These later growths, he says, are developed in the axils of old leaves during the natural decay of the lower part of the stem. They are ultimately set free as fascicles of narrow leaves which, after rootlets are formed at the base of the new growth, sink to the bottom and continue the life of the species. Such structures, which would be analogous to the winter buds of P. obtu- sijolius and P. zosterif alius, have not been observed in P. americanus under investiga- tion, though they may have been overlooked in the changes of water level during the autumn. POTAMOGETON AMPLIFOLIUS. This is an American species distributed quite generally throughout the continent. It forms large patches in the open vegetation but thrives also in close association with P. Robbinsii, Heteranthera dubia, Ceratophyllum demersum, Elodea canadensis, and other plants of aquatic meadows. As a forage plant it may be regarded as one of the best, growing continuously from early spring to late fall or early winter and producing an abundant herbage by reason of its numerous large leaves. The rankest growths have been found in the more quiet waters of Lake Cayuga and "The Pond" at North Fairhaven, at depths of 5 to 7 feet, in a substratum of mud rich in vegetable mold. Propagation is rapid. The dense patches of stems, more or less unbranched, arise in great numbers from an intricately developed subterranean system Cfig. 7). This 262 BULLETIN OF THE BUREAU OF FISHERIES. extensive ramification of underground stems, richly provisioned with starch, remains more or less intact during the winter, carrying at alternate nodes undeveloped shoots which quickly establish new extensions of the plant in the S])ring. Another means of vegetative propagation is found in the detached tips of branches which, after separation from the decaying parent stem in the fall or spring, sink to the bottom and become new centers of growth. New shoots also develop at the nodes of decaying stems and, on separation, sink to the bottom and take root as in the case of detached tips and stems. Besides these vegetative means of growth this Potamogeton produces an abundance of seeds. POTAMOGETON HETEROPHYLLUS. Various forms of this species occur throughout almost all of North America except the extreme north. One of the numerous forms, ^orma terri'Siris Schlectd., is repre- sented in this investigation and all data herein recorded pertain to this plant. It is a so-called land form of Potamogeton and briefly characterized in Gray, seventh edition, as "freely creeping in exsiccated places, producing numerous branches which bear tufts of oblong or oval coriaceous leaves but no fruit." This plant, which grows in the open air after being left entirely uncovered by water, has been observed in two places along the shores of Lake Cavuga — one in a railroad pool 2 miles east of Ludlowville and the other on a sandbar at Myers Point. In each of these places it is interesting to note that gradations in habit accompany the varying changes in habitat. The railroad pool is a particularly favorable spot for the growth of this Potamogeton. It is an artificial pond which has been developed by building a railroad embankment near the foot of the blufl bordering the lake. In consequence, a long, trough-like depression exists between the bluff on one side and the railroad embankment on the other, with water from the lake seeping through and maintaining itself at about the level of the lake. It is a situation especially favorable to the growth of this plant, because the annual withdrawal of the water is gradual, following the natural lowering of the lake level during the summer months. The bottom of the pool is covered with black mud, largely marl in composition, a foot or more in thickness, over which water may rise to the height of 12 to 16 inches. During high-water level in the spring, this Potamogeton grows submerged in the pond with its tuberous rootstalks anchored in the mud (fig. 8). Upon the withdrawal of the water, following the lowering of the lake level in the summer, drought conditions prevail, and then the submerged leafy stems give place to the land forms. Upon the approach of drought conditions the previously submerged leaves die and from the main rootstock or from those arising from the axils of the lower leaves (upper in some cases, Bennett, 1880) runners extend horizontally to a depth of 2 to 6 inches below the surface of the mud. From the fertile nodes of these runners erect axes arise, bearing tufts or rosettes of leaves which cover the ground in great numbers and compete with mosses and small forms of sedges, carpeting the surface of the mud. The leaves of the rosette (fig. 1 2) are unlike the elongated, membranous, submerged ones. They are more rounded in form and coriaceous in texture, and by the presence of sto- mates on the upper surface of the lamina, they are enabled to function as ordinary leaves. POTAMOGETONS IN RELATION TO POND CULTURE. 263 During the season of 1913 the plants which flourished in a submerged condition during the month of May gradually changed their habitat upon the withdrawal of the water during June and became land forms by the first of July. At this time the tuberous rootstocks which perpetuate the plant vegetatively were well developed, and waited only the final stages in the curing process to become the perfected vegetativ^e structures which tide this species over the unfavorable season of growth. On the sand bar at Myers Point, the other station where this Potamogeton thrives, the life conditions are not so sharply marked by the complete withdrawal of the water during the dry season, and the various stages exhibited in the transmutation from aquatic to land forms were easily observed. In water about 10 inches deep the continuously submerged plants developed low bushy stems, with a few coriaceous leaves at the top. In shallower water the plants behaved in the same way, producing bushy, stunted- looking stems, which finally graded into land form with leaves in tufts or rosettes resting on the exposed surface of the sand bar. The rootstocks, which were twisted and con- torted in their effort to become established in the pebbly and gravelly sand bar, were buried from 2 to 4 inches beneath the surface in the rich, black soil of the bar. All of the internodes of these subterranean stems were more or less thickened and often attained a length of 8 to 14 inches. No fruiting plants were found, and this observation is in accordance with the generally accepted opinion that this form of hetcrophyllus is propagated entirely bv vegetative means. Observations on the artificial propagation of this species are recorded in a later chapter of this paper. POTAMOGETON PERFOI.IATUS. The leaves of this plant afford valuable forage material, though the season of growth is comparatively short, the plants appearing late in the spring and dying quite early in the autumn. In the environs of Ithaca this species flourishes in quiet waters either in a substratum of sand at the relatively shallow depths of 2 to 3 feet, or in "aquatic meadows" in a substratum of mud at depths of 3 to 5 feet. The observa- tions of Pieters (1901), in "The Plants of Lake Clair," and of Thompson (1897), in "The Biological Examination of Lake Michigan" extend the range of depth at which this species exists to 12 feet. During the growing season the vigorous underground stems increase rapidly the output of forage material, since a single subterranean system produces a large number of erect, much branched, leafy stems. The experiments of Pond (1903) and Sauvageau (1894) and the obser\'ations of R. B. Thompson (1913) afford evidence of other means whereby the rapid extension of this plant takes place. In accordance with their observations, young branches, which are easilv detachable, float away and rapidly become new centers of growth. In winter the vigorous and abundant subterranean system decays, leaving only the terminal shoots of two or three nodes (Fryer, 1900) to continue the plant the following spring. This plant, therefore, has three important means of vegetative propagation: By readily detached leafy stems, and by extensions of the subterranean system, both of which operate to multiply the plants during the growing season; and by the terminal portions of root- stocks which, remaining in a quiescent state during the winter, establish new plants in the spring. 264 BULLETIN OF THE BUREAU OF FISHERIES. POTAMOGETON CRISPUS. This species, a native of Europe, was recorded in this country by Pursh as early as 1814 (Arthur Bennett, 1901). Since that time it has become established over an extensive area because of the remarkable facility for multiplying itself vegetatively. It is the most abundant Potamogeton in the vicinity of Ithaca, where it flourishes in various habitats — in deep or shallow water, in sand or mud bottoms, and in stagnant pools or flowing streams. It is singularly adaptive in each situation. It has been collected with P. pectinatns growing at depths of 8 feet, in which habitat the intemodes are extremely elongate; it has been found in pools where the substratum is an accu- mulation of debris from ash heaps and dumping grounds; and it is not uncommon in the swifter parts of streams and along the lake shore in sandy situations where the substratum is thrown into ripples by wave and current action. In the latter situation it has always possessed short, stocky stems and a general dwarfish appearance. P. crisf'its grows the year round and spreads with great rapidity. It is propagated primarilv bv "burs," peculiarly distinctive structures to which there is nothing quite comparable in our native species. Alorphologically they are branches, but in the stage most frequently seen they are scarcely recognizable as such members of the plant structure. They have a horny look and a reddish color. The shortened intemodes and thickened persistent leaf bases combine to give the characteristic bur-like appear- ance (fig. 22). POTAMOGETON ZOSTERIFOLIUS. This flat, grass-like species of Potamogeton is not largely foraged upon by aquatic herbivores, yet it appears in greater or less abundance in most ponds and lakes and doubtless ser\-es an important r61e in the economy of life by furnishing support and shelter to the countless small forms which have been found upon it. P. zosterif alius is among the earliest of the Potamogetons to appear in the spring, as well as among the first of them to disappear in the autumn. It flourishes in a sub- stratum of mud in still or running waters, and while it is not adapted to possess the soil so completely as P. crispus, nevertheless it has effective means of perpetuating itself. Mr. A. J. Pieters (1901) remarks that this species, which he has obser\-ed growing in abundance in Lake Erie, may be losing the power to produce seeds. Indeed, during the past season few plants matured seeds in the several regions where they were observed, but all developed winter buds in great abundance (fig. 33). Large quantities of vegetation, that is, the accumulation of the varied and abundant mass that still exists in the autumn, have been hauled up to the surface for examination, and it was both surprising and astonishing to see the vast number of winter buds of this Potamogeton that were entangled among the stems of other plants. It suggests to an extent how well this species accommodates itself to its surroundings. It never forms dense patches of growth, but it often occurs with aquatic plants that form them more or less densely. By virtue of its slender, grasslike habit, it occupies the interstices of the more rank aquatic flora, and it occupies these spaces as simple individual plants, not as erect axes of a complete and intricate subterranean system. The plants are anchored to the substratum by the roots only, which develop from the winter bud, and because of this loose hold in the soil they are readily pulled up. The large number of POTAMOGETONS IN RELATION TO POND CULTURE. 265 plants which have been uprooted appeared always to possess a comparatively simple, erect stem which developed from a winter bud without the ramifications of rootstock which are characteristic of other species of Potamogeton not grasslike in habit. POT.-\MOGETON OBTUSIFOLIUS. This species is apparently an important aquatic forage plant, for its delicate leaves show abundant evidence of larval depredations throughout the growing season. It is somewhat grasslike, yet less stiff and harsh than the preceding species. It is a rare Potamogeton in the flora and has been observed in one place only, Spencer Lake, where it is found in a muddy substratum in shallow waters of more or less swiftness. The plant has a bushy habit of growth, branching widely toward the summit, a habit which tends to produce dense patches of these plants. At one place in the station it grows in such dense masses as to choke up the mouth of a small stream entering the lake. The plants are late in appearing among the other aquatic forms in the spring, lagging behind P. zosterijolius a month or more. The bushy habit of the plant begins to show itself early in the summer, when branches arising near the base of the plant ramifv toward the top until the characteristic bushy habit is attained. Fruit is pro- duced abundantly, but doubtless an equally important structure in the reproduction and distribution of the species is to be found in the large winter buds. These appear on the much-branched stems in great numbers and differ in no essential respect from those of P. zosterijolius except that they are much less stiff. As in the above-mentioned species, they fall away from the parent plant when mature and sink to the bottom. Like P. zosterijolius , too, there is characteristic simplicity in the underground system. The mature plants which have been collected show no tendency to produce ramifications in the substratum, nor any indication of a perennial habit, but the plants become readily propagated vegetatively by means of winter buds or hibemacula. POTAMOGETON FILIFORMIS. A habit sketch of this plant is shown in figure 36. Morong (1893) states that this is a rare species in the United States. One collection only was obtained. The specimens were found early in July near Canoga on Lake Cayuga, where the plant flourished in shallow water and among calcareous rocks along the shore. The plants were short and bushy in habit and bore abundant fruit. In all cases the erect axes developed from a tuberous rootstock which, judging from the numerous erect shoots that grew therefrom, is the common method of vegetative propagation in this species. The tubers (fig. 37) occurred in series of 3 to 5 on the rootstock. Although no opportunity was afforded for studying this plant during the successive seasons, it is deemed worth while to record the obser\'ations of one collection of plants, since this species of Potamogeton is unique in its habitat and promising in the possibility of seed and tuber production. POTAMOGETON PECTINATUS. This species possesses many important characteristics which recommend it to the culturist of aquatic plants. It is one of the most abundant and widespread of the Potamogetons. P. pectinatus is regularly found in quiet waters, though it has a variable habitat in other respects, occurring in a substratum that is sandy or muddy and in waters 86309°— 15 3 266 BULLETIN OF THE BUREAU OF FISHERIES. that are deep or shallow, fresh, salt, or brackish. It is also extremely variable in growth habit. Two of its remarkable lorms which occur in Lake CayuRa and its environs and which Dudley (1886) describes as a slender form" and a gigantic form'' are included in the present investigation of this species. P. f)cctinatus, the species which is common everywhere, is among the first of the Potamogetons to sprout in the spring, making its appearance early in April. Of such plants which appeared early in the season, over a hundred individual specimens were u]irooted to determine the agent of propagation. In all cases these plants developed from tubers which were buried in the mud or sand. Figure 38 shows the general habit of growth from these reproductive structures. The new plant quickly establishes itself by developing simultaneously with shoot formation an extensive subterranean system of stems, which in turn send up leafy shoots in great numbers. By this ramification of the underground stems, P. pectinatus encroaches upon the soil so effectively as to produce dense patches of growth, to the exclusion, in some cases, of other sjDecies of aquatics. The plants bear fruit more or less abundantly, but, in general, tuber formation doubt- less equals or surpasses seed production. Tubers of various size occur, the size being dependent, more or less, on the nature of the environment. The largest and finest specimens were found at North Fairhaven in the quiet waters of Sterling Creek, where P. pectinatus forms a part of an equatic meadow renowned for its luxuriance of vegetation. These large tuber-bearing jjlants grow in the rich, muckv substratum at a depth of 6 to 10 feet in association with Elodea canadensis, Myriophyllum spicatum, Ceratophyllum detnersum, Utricularia vulgaris var. .1 mericuna, Nymphea adiena and other Potamogetons, such as amplifolius and zosieri- jolius. In this situation the plants are rapidly propagated from the tubers. On June 21 several specimens were collected which illustrate the complete cycle of tuber forma- tion. Plants retained intact the old tubers, the new shoot — a tall, leafy, erect axis bearing in some cases a floral spike — and the new rootstocks bearing tubers. On many plants in this most favorable environment the tubers were greatly in excess of the matured fruits, and often the only reproductive structures. The plant dies down early in autumn. In October attempts were made to collect underground stems to determine, if possible, a pereimial habit in this region. Only portions of the rootstock were secured, but in every instance disorganization had progressed to a considerable extent. The appearance of the tuber in the spring, when many of the plants were uprooted and observed with shoots growing from them, indicates a complete and natural separation from the parent stem, probably in the autunm. It may be inferred, then, that the tubers are the only vegetative structures that do survive the unfavorable growing season. The slender form described by Dudley (1886) was found still occupying the same region in Cayuga Lake where it was observed by him many years ago. The plants a 1007. var. (?) with slender eloncated stems ( i lo i ■ imeters) ; nodes remote, as are the whorls of the spike, whose peduncle is usually over onc-Iourth meter long. Leaves few and slender, plants sometimes proliferous. Xear the lighthouse. Ca>'uca L. Dr. Robbins "found no parallel for this remarkable form." in his own observations. Dudley. William R.: The Caj-uca Flora. Bull. Cornell Univ. (Science), p. 107. 6 1008. var. (?) a gicanticfonn Browingin deep water northwest and northeast of the ligbthouse. Ca^'uga L. Not yet found in flower or fruit, though examined more or less frequently during 10 years past. It is frequently proliferous, especially if detached. It grows in banks, the plumelike bushy tops reaching the surface of the water. The leaves and sheaths are similar to P. pcclinaluj. except in length. Dr. Robbins remarked that he had " nothing that comes nc.ir to it in length of leaves— usque ad 10." Stipules are usually much shorter than in P. ptctinalus. Specimens were obtained in 1874 from 4 to jM meters long. This lorm was also noticed by Mr. H. B. Lord, probably somewhat earlier than 1874. hoc. cit. POTAMOGETONS IN RELATION TO POND CULTURE. 267 grew in banks in sand and silt bottoms at a depth of 5 to 7 feet. They were quite unmixed with other aquatics. In July and early August the long heavily fruited spikes floated in dense masses at the surface and gave to these areas of the water a characteristic brown look. Proliferations were not found on these plants during the summer; fruits, however, were more abundant than on any other form of pectinaius. The gigantic form of pectinaius grows in deep water. Plants 8 feet long are com- mon, although many average but 5 feet at the end of the growing season. This form grows in a substratum of sand and silt at depths varying from 6 to 1 2 feet in a region of the lake exposed to a more or less constant sweep of the wind. The plants, there- fore, which grow practically to the surface of the water, are subjected at times to vigorous wave action. Altogether these environmental conditions favor a growth of remarkable luxuriance. The plants grow in banks, and so thickly as to preclude the possibility of encroachment by other forms of vegetation, though in shallow places, where the growth becomes sparser, a few scattered representatives of P. crispus, P. perjoliatus, and Heteranthera duhia occur. This form of pectinaius begins growth early in the spring. In May, 1913, the plants already approached the surface of the water. On June 21, 1913, a plant bearing a single floral spike was found, although in several collections made thereafter neither flower nor fruit was obtained. This appears to be the first record of a floral spike on this form of pectinaius. From the collections made in November a few tubers were found on the tips of the foliage sprays of the plants that were uprooted from their natural moorings, although they were found more commonly on sprays that were floating in the drift. This latter obser\'ation is an agreement with Dudley (1886), who observed and described this form in Lake Cayuga. No rootstocks were secured, since attempts to uproot the plants at such depths with a grapple resulted always in breaking the stem just short of the subterranean system. This appeared to be embedded firmly and deeply in the substratum, at least more deeply than the length of the grapple teeth, which measured 4 inches. However, the bases of the erect stems, the parts which develop just above the rootstocks, possessed remarkable examples of proliferation. Thickened runners, more or less contorted, arose from leaf axils at the bases of the erect stems (fig. 50, A), terminated by large, elongate tubers. The bases of the stems were hard and woody, more especially so in the regions where they became detached from the underground system. This condition suggests a continuation of the woody structure in the subterranean parts. It may be inferred perhaps, from the general habit of the plant and the attendant conditions of growth, that the rootstocks are per- ennial, and that the basal runners, which bear in abundance large tubers and green shoots, are the chief propagative structures of this form of pectinaius. POTAMOGETON ROBBINSII. Although this Potamogeton is less well known than the other species, it is destined to be regarded as an important aquatic forage plant, first, because it is very prolific, and, second, because the foliage is very generally eaten. The habitat of this species, where it has been under observation, is not unlike that of P. amplijoiiiis, with which it is often found in association. It has been observed in the quiet waters of lakes and ponds at depths of 3 to 5 feet in a substratum of rich, black mud. The stems ascend from a somewhat creeping base and branch profusely in a more or less tvyo- 268 BULLETIN OF THE BUREAU OF FISHERIES. ranked arrangement, forming large, broad, flat sprays of foliage, which often cover the bottom in large patches. It is the rarest of all Potamogetons to fruit, at least in the situations where it was observed, but because of the tendency to branch profusely propagation is readily effected. The branches, especially those whose intemodes remain short, become thickened and hardened through the storage of starch, and when detached function as propagative structures. This enlargement and induration may occur also at various points along the main axis that bears the propagative branches, so that the final dismemberment of the whole plant provides enormous possibilities in the niultiiilication of the species. Dismemberment may occur in the autumn, but the plant is hardy, and this natural separation of parts may be deferred till spring, then long rootlets develop at the nodes and establish the plant at once. The plant is tardy in beginning its growth in the spring, but this tardiness in growth is obviously advantageous to a plant that propagates mainly by vegetative means in the manner of this species. Moreover, a very material advantage accrues in that the full and complete foliage of this Potamogeton apjiears late in the season when many other aquatics, including Potamogetons, show signs of decay. Growth occurs during the winter. It is not great, however, and manifests itself only in a slight elongation of the branches, producing fresh, green tips of foliage, which are foraged upon by aquatic herbivores almost as fast as the leaves appear. SUMMARY OF CULTURAL FEATURES. The Potamogetons which yield important forage products fall into two groups: Those which produce abundant herbage in their leaves — P. amerkanus, amplifolius, perjoliattis, crispus, and Robbinsii — and those which develop a large supply of starchy food products in the tubers and tuberous rootstocks — P. pectinatus, fUijormis, and heierophyllits. The species which grow best in the currents of streams are P. americanus and obtusijolius; in deep water, P. pectinatus, especially the slender and gigantic forms of Dudley; in calcareous regions, P. helerophyllus and filiformis; in exsiccated places, P. hcierophyltiis. The species appearing early in the spring are P. americanus, zosterif alius, pectinatus, heierophyllus, crispiis, and amplifolius; those growing late in the autumn and continu- ing throughout the winter are P. crispiis, amplifolius, and Robbinsii. Abundant fruit is produced in P. perfoliatus, obtusifoliu^, and filiformis, and on pectinatus in most situations. \'^egetative reproduction occurs freely in all species. The important vegetative structures are: Winter buds or hibemacula in P. obtusifolius and zosterif alius; modified branches in P. crispus and Robbinsii; tubers in P. pecti- natus and filiformis: tuberous rootstocks in P. heterophyllus, and subterranean buds in P. anwrieanus, amplifolius, and perfoliatus. NATURAL AND ARTIFICIAL PROPAGATION. The natural propagation of Potamogetons has been touched upon in a general survey of life conditions, and it has been seen that these plants propagate freely by means of various vegetative structures. At this point it is desirable to consider this method of propagation in greater detail, and to present data which will afford a means of com- parison between the general seed habit and the tendency to produce vegetative propa- gative structures. POTAMOGETONS IN RELATION TO POND CULTURE. 269 PROPAGATION BY TUBERS. A conspicuous method of vegetative propagation is seen in the development of plants from tubers in P. pectinatus and P. filiformis. The tuber-forming habit of pectinatus has been described by Irmisch (1858), who carefully worked out the morpho- logical details of the tubers in terms of the ordinary stem structure. His figures illus- trate the development of tubers on detached parts of leafy stems, on the erect axis, and on the underground stems. It is not clear from which forms of pectinatus these drawings were made. In general, however, they bear a close resemblance to our most common representative of pectinatus, though no hint of the variability in this species is given beyond the fact that some plants were collected in deep water, and that the tubers were varied in shape, some being more cylindrical than others. The work of Sauvageau (1894) confirms the observations of Irmisch as regards the tuber-forming habits of pectinatus, but this investigator also makes no allusion to the remarkable forms that exist in this species. His drawings, moreover, are, as he states, modifications of those by Irmisch. Both of these workers in this field recorded the time of tuber formation to be in the autumn. Jepson (1905) suggests an earlier development for those on the rootstock and the erect stem. He says: "The slender threads which develop one, two, and even three tubers at the end, are not only borne on the horizontal rootstocks and on the soil at the bottom of the ponds, but are also produced on the upright stems, and at the end of the season on the uppermost leafy portion." Regarding the presence of tubers on rootstock, stem, and spray, the present investi- gation is confirmatory. Tubers have often been observed on all these parts of the plants. Additional figures and observations relate more especially to the season in which they occur and to their artificial propagation. Collections of plants made on the 15th of May, 1 91 3, and thereafter throughout the growing season, show the presence of tubers in great numbers on the proUferating shoots of the rootstocks. Many of these tubers are well grown in May, though others subsequently arise on the extensions of the subterranean system which develop after this time. Figure 41 represents the basal part of a small immature plant of P. pectinatus col- lected in shallow water June 20, 1913. Many plants at this time were more nearly mature and bore larger tubers, but it seemed desirable for illustration to select a small plant because in such all parts may be preserved intact during the collection of material, a task that is attended with considerable difficulty when the plant has attained a large size and great complexity of parts, especially in the subterranean region, where the underground stems are exceedingly brittle and tender. This figure (fig. 41) illustrates the general sequence of growth in what may be termed the typical vegetative life cycle of the plant. The order of development is as follows: The production of a leafy, erect shoot (C) from the tuber of the preceding season (A) ; the growth of the horizontal axis or rootstock (D) ; and the production of the stolon-like branch or runner which in turn bears a tuber or tubers at the end (B). As the season progresses the tubers become solidly packed with starch in sufficient amount, apparently, to bring the plants developed from them to a very advanced stage of growth, at least to render them quite independent of the soil for a considerable length of time. Figure 45, B illustrates the typical condition in this respect when tubers sus- pended in aquaria without contact with the substratum produce the future propaga- tive structure. Thus the continued dependence of the plant upon the stored starch in 2 70 BULLETIN OF THE BUREAU OF FISHERIES. till' tuber would seem to be advantageous, especially if growth occurred under untoward conditions. The tubers, hardened by the great quantity of starch that is packed into the tissues, normally pass through the winter in a donnant slate. This, however, is quite easily disturbed, and by supplying continuously ordinary room temperatures the tubers may send forth shoots as early as October. Figure 45, noted above, illustrates such a response to growth conditions, the plant having been developed between the dates of October 22 and December 20. The propagation of tubers in aquaria has shown that when tubers occur in twos, for example, figure 40, the larger one develops the shoot. The smaller one has never been seen to sprout unless by chance it became detached. In that case it developed an individual plant. It has been frequently observed that plants of this species when propagated in aquaria never attain their full size or vigor when deprived of a soil sub- stratum, an observation that is in accord with the results of Pond's (1903) experiments on rooted aquatic plants. The remarkable versatility of P. pectinatus as regards the origin of tuber-bearing runners has been clearly shown by Irmisch (1858). There is, moreover, in each of these situations, on rootstock, stem, and spray, a considerable variation in size and number of tubers. For example, an underground stem or rootstock may develop them at the ends of slender, stolon-like branches which arise from the axils of fertile nodes as shown in figure 43. These have been found singly or in pairs, large or small, depending upon the richness of the substratum and the size of the plant. Again, the rootstock itself may be terminated by tubers which occur singly, in pairs (fig. 42), or in threes (fig. 39). Plants bearing rootstocks of this character have been collected at various times during the growing season, and from each collection the specimens have shown comparatively short underground stems without other tuber-bearing structures. Some rootstocks have shown no tendency to produce tuber-bearing runners or tubers at the end of the horizontal axis, but send up a succession of leafy shoots from the fertile nodes. It is suspected, however, that had such plants been undisturbed tubers might have devel- oped, especially since at the base of these upright shoots there was always a bud, either latent or showing a tardy development. In autumn pectinatus develops tubers on the leafy spray. They are generally smaller than those which occur on the rootstock, but quite conspicuous because of their pale, yellow color. They are borne singly or in pairs at the ends of runners that are bright green and stouter than the stems from which they arise (fig. 44, B, C). These structures are readily distinguishable about the time the plant begins to show signs of decay. They may occur on attached or detached parts of the plant. The remarkable prolificity of these sprays is a characteristic of this species. Repeatedly detached parts of the leafy spray have been placed in aquaria and tubers have been developed in abundance until the spray became completely disorganized. It is interesting to note that when this species grows in the currents of the stream the tendency to form pro- liferations on the leafy spray is conspicuously lessened, although portions of these plants when caught in the drift and carried to quiet water readily produce them in the new environment. For the most part tubers are more numerous on sprays devoid of fruiting spikes, although exceptions are frequent. In examples of this kind, figure 44, B, C, shows the POTAMOGETONS IN RELATION TO POND CULTURE. 271 origin of tube-bearing structures, one arising near the base of the peduncle, the other solitary from the axil of a leaf. Figure 44, A, is a detail of such a spray showing the usual character of the runner. Runners arise also on the lower parts of the stem (Irmisch, 1858). These, like many on the tips of the spray, may develop so late in the autumn that tubers never mature. What their fate is during the winter can only be conjectured. It is a fact, however, that when placed in aquaria they continue to grow slowly and eventually produce small tubers, or remain for a time in a quiescent state and then send forth leaves and other runners. In the gigantic form of pectinatus (Dudley, 1886) the tubers are elongate and large in size. They are born on runners at the bases of the stems just above the substratum of mud, and are therefore several feet beneath the surface of the water. Figure 61 shows the entire leafy axis with a tuberous runner attached at the base of the stem. This is the normal position for what appears to be the chief propagative structure of this form of pectinatus, and the usual condition at the approach of winter. The runner is seen in detail in figure 50. The tubers are yellowish in color, and when stripped of scales, which envelop them at this season, appear as in figure 51. The remainder of the runner is dark green in color, more or less contorted and tuberous, and hardened throughout by storage of starch (fig. 50, B). Secondary runners bearing tubers (fig. 50, I, 2) are additional features in what withal is a remarkable propagative structure. Peculiar tuberous intemodes, transition stages, perhaps, in the formation of tubers, appear frequently and characterize the more hardened and resistant portions exclusive of the terminal tubers (fig. 52). On germination a leafy shoot and runner are pro- duced. Figure 55, an illustration of a similar feature repeated in a series, was devel- oped in an aquarium from the terminal tuber of a small runner. It illustrates how resourceful in the propagation of this species so small a structure may become. Young, green, leafy shoots arise from the fertile nodes of the runner (fig. 50, D) and doubtless function in perfecting the propagative structures of this persistent part of the plant, for at this time^that is, in the autumn — the leaves of the main axis begin to disorganize. The young shoots retain their greenness through the winter, remaining in a quiescent state meanwhile, and produce the main axis of the new plant the following spring. When these structures are transferred to aquaria, they pass through a winter-rest period, a period which is less easily disturbed, however, in this form of pecthmtus than in others of the same species. Extreme plasticity is character- istic of various portions of the runner. Fertile nodes produce either tubers direct, or leafy tips, or runners, any one of which may in turn produce a runner. The tip of a secondary runner may produce a leafy shoot (fig. 53), and a tuber, instead of elongating its axis in the natural way, may develop precociously a reser\'e bud which produces the leafy stolon (fig. 54) . As in pectinatus generally, the detached sprays of the gigantic form show a greater tendency to produce tubers than the attached ones. Likewise the runner is the im- portant structure which bears them. Such tubers may become very numerous. As many as 15 have been counted on a single plant (fig. 62). Detached portions of the plant bearing tubers float away in the drift, from whence they may or may not find a favorable place of growth in the spring. The tuber-bearing runners developed at the bases of the stems rarely become loosened from the tangle of vegetation at the bottom and must therefore repopulate the area year after year, encroaching but slowly on the surrounding region. 272 BULLETIN OF THE BUREAU OF FISHERIES. P. filiformis represents a tuber-forming species which produces these propagative structures apparently in the manner of P. pectinatus. Since material was collected but once during the summer, no definite data can be recorded regarding the details of tuber formation beyond the fact that the plants develop from tubers, as the collected materials show (fig. 36, 37), and that these tubers, whether they occur singly or in a series of two or more, have a likeness to those of pectinatus, in size resembling the common form and in shape approaching more nearly the deep-water fonn. In details of structure the tubers of filiformis are similar to those of pectinatus. Judging from the general habit of the plant it seems fair to assume that the tubers have arisen in the same way and that vegetative propagation would depend largely upon them. PROP.AG.ATION BY TUBEROUS ROOTSTOCKS. The vegetative structures of P. heterophyllus tcrrestris are illustrated in figures 10 and 1 1 . Morphologically they are a series of more or less shortened and hardened internodes richly provisioned with starch. They are borne at the temiinal portions of the underground stems. Well-developed buds, the incipient, erect axes, occur at alter- nate nodes of the structures, while the intervening nodes remain sterile, as in the case of undifferentiated rootstocks. Figure 8 represents a typical plant collected early in May. At this season the plant is still submerged. The tuberous rootstock of the pre- vious year sends up young, erect shoots from the fertile nodes, and extends the growth horizontally by an elongation of the terminal bud to form the new rootstock. The underground stems acquire a distinctly tuberous appearance very early in the summer. At jMvers Point, where the collections were made frequently, the tuberous character became apparent at the time when drought conditions began to prevail in the pools; that is, when the water level was reduced to such an extent that the sub- merged, leafy shoots gave place to the later-formed, erect shoots topped with tufts or rosettes of aerial leaves which rest upon the mud. Figure 12 represents a plant of this kind. By comparing the plants shown in figures 12 and 16 the origin of the tuberous rootstocks is clear. In figure 16 tuberous structures appear at the ends of the new underground stems, B and C. This tendency to produce the tuberous growth may appear earlv when the plant is still submerged, though it may be deferred till drought conditions prevail, when the new type of leaves forming the rosette above the ground function to produce the abundant storage of starch which is found in the mature tuberous rootstocks. Some underground stems, throughout the growing season, continue to produce internodes nontuberous in structure (fig. 9), but they arc exceptional rather than the rule. The tip of the rootstock that is destined to become tuberous generally shows this character very early. The internodes at the end do not elongate in the usual way, but appear serially in a more or less bead-like form (fig. 10 and 11). Figures 13 and 14 represent the tuberous rootstocks partially developed. Figure 10 shows a fully mature one. These structures, and many others in similar stages of development, were col- lected in July and it is interesting to note that while some arc only approximately mature others are fully so thus early in the season. In November all evidences of other plant parts have disappeared and the tuberous rootstocks only are left isolated in the mud, where they remain in a quiescent state through the winter. A typical structure, as it POTAMOGETONS IN RELATION TO POND CULTURE. 273 appears at the beginning of the winter, is seen in figure 11, although variations in the length and thickness of internodes are not uncommon. Tuberous rootstocks have been transferred to aquaria, where the growth has cor- responded exactly with that exhibited in the natural habitat except in one respect, the development of aerial tufts of leaves. But the explanation of this omission in the cycle of development is clear, since the plants remained submerged in the aquaria. The period of desiccation not- having been interpolated, it is assumed that the tuber formation progressed in a natural manner for the species. Figures 16 and 17, drawn from aquarium specimens, show how in the purely aquatic phase of its existence the natural habit of growth and reproduction in this Potamogeton is reproduced under artificial cultivation. PROPAGATION BY SUBTERRANEAN STEMS NOT TUBEROUS. Among the species studied, P. perfoliatus, P. amplijolius, and P. americanus are propagated in this manner. The plants are carried over the winter by means of the terminal portions of underground stems, which are generally stouter than the ordinary ones and which bear conspicuous scaly buds. These buds are the incipient shoots from which the elaborate plant structures of the following season are developed. Sauvageau (1894) has figured this propagative structure for P. perfoliatus as he found it at the approach of winter. He states that the entire plant dies in autumn, except a few inter- nodes which bear the buds for the continuation of growth in the spring. In figure 18 is represented a portion of an underground stem that survived the winter and pro- duced the first few internodes of growth. The scales on the part that lasted through the year are distinctive in appearance. They are larger and looser than the ordinary ones, black in color, and leathery in texture (fig. 18, A.) In P. amplijolius perennial parts are also found in the underground stem. Figure 7 represents the characteristic features of such a structure at the beginning of the winter. The young, erect shoots A, A, A, wit partially unfolded leaves at the tips, pass the winter unchanged and serve to promote rapid growth in the spring. The buds terminating the horizontal stems remain latent through the winter and on unfolding in the spring push out in all directions through the substratum. In these ramifications a subterranean system of interlocking stems and roots is developed that fixes the plant with exceeding firmness in the soil. In P. americanus vegetative propagation is accomplished by subterranean scaly buds which generally grow in pairs at the end of the rootstock (fig. 4 and 5). The general structure of the bud resembles that of P. perfoliatus. It is an incipient shoot, possessing a succession of very short internodes and young leaves, with scales surrounding the whole axis. A small portion of the rootstock generally remains attached to the buds and persists through the winter. PROPAGATION BY WINTER BUDS. The winter buds afford the only means of vegetative propagation which have been observed for P. zosterifolius and P. ohtusifolius. These structures develop at the ends of the shoots. The terminal internodes remain short and, becoming completely cov- ered by closely overlapping leaves and stipules, form a hard, compact, cone-like bud. 2 74 BULLETIN OF THE BUREAU OF FISHERIES. Such buds become conspicuous during the month of August. Later when they are mature they easily fall away from the parent axis, which thereafter dies down com- pletely. Being heavier than water, the buds sink to the bottom and by the middle of October thev have either disappeared or have become entangled in the accumulations of Elodea, Myriophyllum, Ceratophyllum, etc., which still remain intact. In the dis- organization of this mass of vegetation, a gradual settling of the entangled buds takes place and they eventually find lodgment with the others in the substratum of mud, where they remain in a quiescent state till early spring. Such buds may properly be called hibemacula, since they pass through the unfavorable winter season in a state of rest. The general external aspect of the winter buds is seen in figures 33, 63, and 64. In size and form the two buds are quite similar but the leaves of obtusifoHus are less stiff and harsh. In the internal structure of the bud (tig. 34) the typical branch-Uke character is apparent with the young leaves closely crowded toward the tip. Plants of both species have been reared in aquaria by anchoring the buds in sand or mud. The latter operation is not necessary, however, since mature buds sink naturally to the bottom, but it was a precautionary measure against the disturbance of buds under obser\'ation in aquaria. The plants of zosterifolius thus propagated did not bloom, but produced winter buds; those of ohtusifolius bore flowers and fruit early in August. During the winter the loose leaves on the outside of the bud decay, but, on the whole, the entire bud is well preser\'ed. This resistant character is more especially true of zosterijolius, in which many of the enveloping leaves of the bud persist long after the new plant has become estabhshed. In the spring the first sign of growth is manifested by a spreading of the inclosing leaves. Then follows the development of roots from successive nodes (fig. 35) and the elongation of the inlernodes at the tip of the bud. This elongation carries the young leaves forward and upward, and in a short time the general habit of the plant becomes apparent (fig. 65). The various stages in the growth of the bud in the spring are, in so far as they have been observed, similar in those two species of Potamogeton, except that obtusifoHus lags behind zosterifolius. PROPAGATION BY BURS. P. crispus is the single example of such vegetative propagation. The first evidence of propagative structures by means of which the growth of this species is rapidly extended became noticeable early in May. At that time the so-called "burs" (fig. 22) made their appearance. They were enormously abundant, appearing in the axils of nearly all the leaves. Many of them became fully mature by the middle of the month, especially those which developed in pools of standing water where the daily temperature of the water was comparatively high. In the colder waters of spring pools and of the open lake these propagative structures, like the flowers and fruit, were retarded in development, maturing about two weeks later. As the summer advanced the development of the burs decreased until by the middle of July only scattered individuals were to be found. As a rule, the burs occur in the axils of the leaves. They may, however, terminate the growth of the axis (fig. 30). In this latter position they may occur in pairs (Savau- geau 1894), often with a flowering spike. They may develop from the rootstock POTAMOGETONS IN RELATION TO POND CULTURE. 275 directly (fig. 31), though this occurs but seldom. On the maturity of the bur detach- ment from the parent stem is an easy and natural process. The tissue just below the pointed base of the bur becomes softened and the burs fall away, either by their own weight or by accidental contact with other objects. On reaching the bottom, anchorage in the substratum is facilitated by the peculiar shape of the bur, a sharp- pointed, spindle-shaped structure that is heavier than water. A rest period occurs before germination takes place. This rest period is apparently a varied one, depending on the season when the bur is matured. Those which matured early in the season, in so far as it could be determined, germinated in the fall, and in October bore shoots from 6 to ID inches long (fig. 59). Those maturing late passed the winter in the quiescent state and germinated early the following spring. The slender, spicular burs (fig. 21) described by Irmisch (1858) and by Sauvageau (1894) were found more or less commonly in the axils at the base of the erect stem, and always few in number compared with the stouter form. It is interesting to note in this connection that these spicular burs appeared more abundantly on the so-called "state" of P. crispus, a plant with flat, not undulate leaves, said to be a young state of crispus (Fryer, 1900). In one of the spring pools from which collections were made the spicular buds predominated on what appeared to be matured plants of this flat- leaved form. The plants were never so vigorous looking as those in the other situations, and the appearance of the spicular burs upon them may be explained by differences in habitat. Generally they appear to be poorly conditioned plants, and from observa- tions on their development it would seem that they are a star\'ed state of crispus rather than a young state. The development of the large bur (fig. 22), which Sauvageau (1894) described in part, has been observed in the field and in aquaria throughout the various stages, from its beginning as a small branch to its completion as a mature bur. Since the steps in the formation are essentially the same under natural or artificial conditions, observa- tions will be presented on the material under control in the laboratory. Vigorous looking plants were collected in the latter part of March and anchored in a soil substratum in aquaria with running water. Cuttings also were used, some of which were anchored in the soil and others allowed to float on the surface of the water. Three weeks later, short, stunted-looking branches appeared in the axils (fig. 26, A). They exhibited at once a noticeable thickness of the axis and later the peculiar denticulate appearance at the base of the leaves (fig. 22, a). When the diameter of the branch had become considerably augmented and the denticulate margin conspicu- ous, disorganization of the leaves commenced from the distal end and proceeded toward the base. Disorganization ceased at the tip of the denticulate base (fig. 22, a, i). By this time the basal portion of the leaf was hardened, thickened, and homy like the axis, and the entire structure presented the characteristic burlike appearance. Figure 60 shows several small-sized denticulate burs in various stages of development. Essentially the two kinds of burs are similar, differing only in certain minor details. In the bur shown in figure 22 the leaf bases are large and always denticulate, the buds in the axils are relatively small, and the intemodes are short. In the spicular bur (fig. 21) the opposite is true. The leaf bases are small and spinous with a smooth margin, the buds are well developed, and the intemodes are comparatively long. A difference 276 BULLETIN OF THE BUREAU OF FISHERIES. between them is also apparent in the time of occurrence and in position on the stem. Irmisch (1858) recognized a disparity between them and suggested a difference in origin, though he was not able to determine this for both forms. The spicular burs he found originating from the axillary buds of decaying, floating stems in autumn. The den- ticulate ones he found always mature and detached from the parent stem in muddy bottoms of pools. Sauvegeau (1894) describes and figures both forms of burs, giving their origin as well. j\Iy obser\'ations, however, are not in full agreement with their representation on the stem as expressed in Sauvageau's figures. According to his illustrations, both forms are abundant on the same branch and at the same season of the year. This has not been found to be the established order in vigorous and healthy- looking plants. Numerous collections of P. crispus indicate that when the denticulate burs are abundant — that is, in the early part of the growing season — the spicular burs are scarce, and if present on the same stem they are sparsely represented at the base of the axis. In every case the large denticulate bur seems to be the product of strong and vigorous-looking plants, and the spicular bur a result of poorly conditioned ones. That the spicular bur is a weakling would appear to be borne out by obser\'ations on their development. When grown in aquaria they have been found on sickly-looking plants and when germinating burs have been deprived of their vigorously growing shoots, small shoots bearing spicular burs have replaced them. In this instance a dis- turbance of the natural trend of growth would be the occasion of their formation. When the spicular burs germinate they produce shoots bearing leaves not crisped, but narrow and flat (fig. 25). The internal structure of the bur is fundamentally like that of the ordinary stem. No new features appear in the tissues of any part of the bur, but starch grains are present in such great quantities that the cells become distended with them. In the fully developed bur (fig. 71) the cells become so greatly expanded that the air cavities are practically obliterated. It is to these distended cells so compactly stored with starch that the hardened, indurated character is due. The accumulation of starch in the bur furnishes an abundant storage supply for rapid growth, after a rest period of greater or less prolongation, depending upon the time of formation. Burs formed early in the summer maj- germinate early in the fall, or, like those of later development, pass the winter in a quiescent state. Figure 23 shows a stage of germination which is usual in the early spring. It is obvious from the general appearance of the shoots that burs of this character passed the early part of the winter in the resting stage. At the same time burs much more advanced in stage of growth (fig. 32) arc frequent, and it is assumed that these are comparable to burs that germinated in the fall (fig. 59) and grew but little during the winter. In aquaria a varia- ble rest period is common. Under these conditions burs have been germinated after periods of six weeks and of three months. In the germination of a bur there are as many possibilities for the production of stems as there are axillary buds on it, although usually not all of the buds germinate. The greater number of burs bear but one shoot eventually, but several may begin growth and produce short shoots (fig. 23). By experiment it has been found that when a bur is broken into bits with one bud per node, each bud will produce a shoot. In the development of a plant from the bur, progress in the growth of a shoot manifests POTAMOGETONS IN RELATION TO POND CULTURE. 277 itself first bj' the establishment of an erect axis, from which very soon a subterranean system arises in the manner shown in figure 27. By further extensions of these axes the number of branches is greatly augmented and the capacity for multiplication greatly increased. P. crispus, like most of the Potamogetons, propagates readily by detached stems. Many of these have been picked up in the drift along the lake shore where under favor- able circumstances some, doubtless, find lodgment and establish new centers of growth. Besides, in the spring there have been found leafy axes which, while still remaining attached to the parent stem, lie prone upon the muddy or sandy substratum and, be- coming rooted at the nodes, send up a long series of erect stems (fig. 20). In this manner P. crispiis combines the rapid growth from stolons with the nonnal spread of the subter- ranean system and forms an effective means of possessing the soil. The large number of burs which are developed indicate that they are the chief source of distribution in this species. Some plants doubtless develop from seed, though they can not represent any great number of the whole since comparatively few seeds mature. To obtain some data on this point a large number of 3'oung plants were pulled up and to the most of them a bur was attached, an observ^ation which shows that, for the region at least, this structure was the chief agent of propagation. From the standpoint of prolificity, P. crispus represents a desirable species for cultivation. It remains to be shown that this abundant herbage is of importance in the economy of aquatic life. Data relative to this are recorded under the heading "Economic aspects of Potamogetons." PROPAGATION BY FRAGMENTS OF STEMS. In P. Rohhinsii the propagation occurs exclusively by vegetative means, depending upon a more or less complete dismemberment of the plant. This breaking of the plant into propagative structures does not take place at random, but occurs at very definite points throughout the leaf-bearing part of the plant. At intervals along the axes of the stems, a few intemodes develop which are very short, and in them starch is stored so abundantly that they become hardened and stiff and noticeably thickened in diameter. At the limits of these indurated regions where the stems appear constricted, the tissues soften when the structures are mature, and dismemberment becomes a natural operation. The process of separation is similar to that which is met with in P. crispus and which causes the detachment of the bud from its parent stem. Besides the main axes of the plant which break up into many potential units, there are also numerous short, axillary branches which possess the characteristic feature of the propagative structure. The intemodes are likewise short and stiff and conspicuously augmented by the deposition of starch. Moreover, they are always provided with a growing terminal bud, a feature which facilitates rapid propagation. When an axillary shoot becomes 6 or more inches long it behaves like the main axis of the stem eventually breaking up into several propa- gative structures. In figure 67 is represented a single branch showing the constricted appearance which distinguishes a stem bearing more than one propagative structure. In the spring, often before a general dismemberment of the plant occurs, very long, white rootlets are developed at the nodes (fig. 57). These rootlets serve to anchor the new growth, whether it be an attached part of the plant or a scattered fragment of the stem. The provision for the initial growth in these fragments of stems lies in the storage 278 BULLETIN OF THE BUREAU OF FISHERIES. of Starch within the tissues. Starch is so abundant that the air cavities are considerably reduced by the distension of the cells (fig. 71). In portions of the stem where the tissues are not obscured by the deposition of starch, it is seen (figs. 69, 70) that mechanical tissue is scattered through the stem in greater abundance than is common in the other Pota- mogetons, serving to support the heavy sprays of foliage and to give the rigidity of stem which is characteristic of this species. In P. amplijolius the tip ends of the branches function as propagative structures in a manner similar to P. Robbinsii (fig. 58). These structures appear in the autumn devel- opini^ only at the tips of the branches. The internodes are short and thick and densely packed with starch. At the end there are a few partially unfolded lea\es which con- tinue to grow slowly or, at least, remain green all winter. These rapidly expand when the roots develop in the spring and the eiitirt' structure forms an effective and rapid means of propagation. PROPAGATION liY SEEDS. While the main purpose of this paper is a consideration of the vegetative means of propagation, yet it is important by way of comparison to present such data as are available on the propagation of these plants by seeds. In reviewing the literature on the seed germination of Potamogetons, it appears that Irmisch (1858) and Sauvageau (1894) have made the only contributions of importance." Irmisch figures the germinating seeds and two small seedlings of P. natans but otherwise gives no data concerning them. Sauvageau found that P. crispus, perfoliaius, and pectinatus germinate in less than a year and that P. natans remains dormant three or more years. No figures accompany his account of their behavior. In the course of the present investigation additional observations have been made on P. pectinatus and P. americanus . The seeds of both species were gathered in October and kept in cold storage through the winter. On January 24 seeds of each kind were placed in aquaria and kept at ordinary room temperatures. On February 14, the seeds of pectinatus began to germinate, but this process was very irregular, extending over a period of three or more weeks. These seedlings lacked vigor and nothing came of them. On March 15 other seeds of the same species were taken from cold storage and placed in aquaria as before. In this later planting germination was more uniform, the majority of seeds sprouting within a few days of each other. Subsequent growth was rapid and vigorous. It appears from the behavior of the seeds in the two experiments that the later planting is advantageous. Figures 46 and 47 represent seedlings of the second planting 3 and 5 days old, respectively. Figure 48 represents a seedling of the same species about ID days old, and figure 49, one about 3 weeks old. The seeds of P. americanus planted on January 24, showed no signs of life till May 5. Those of the second planting germinated between June 13 and 15. In this species also the later planting proved to be more successful. Figures 2 and 3 represent seedlings, respectively, 5 and 14 days old. When the seedlings were about 3 weeks old they were transplanted and kept in outdoor aquaria with running water till October. Figure 4 shows one of these seedlings which produced winter buds during the latter part of the growing season. These winter buds described in a preceding chapter are the vegetative 1 1n a recent publication by Esenbcck the se> I 13 (0) S I 3 I (") 3 6 II a I 6 3 (") 4 IS 8 (•) 3 S 35 34 71 86 3" 43 o Imperfect record. » This plant bore one sterile spike. The fertile spikes of P. pectinatiis produce, in general, from 10 to 15 seeds. The tubers occur singly, in pairs, and in threes. Bearing these possibilities in mind, the tabulation of P. pectijiaius indicates a close approximation to equality in the produc- tion of seeds and tubers. The small number of plants from which the data were collected is an objection which could be justly put forward, yet the results conform in general with field observations in restricted areas where the common form of pectinatus POTAMOGETONS IN RELATION TO POND CULTURE. 28 1 predominates. Propagation by tubers is, as we have seen, the more rapid method and the one which produces a luxuriant foliage early in the growing season. In view of the observations and experiments, it is clear that in any project in which the propagation of Potamogetons is an important feature, success will be measured by adherence to the general principle that vegetative reproduction is the dominant mode of increase in the genus Potamogeton. ECONOMIC ASPECTS OF POTAMOGETONS. In the study of the various phenomena attending the propagation of Potamogetons opportunity was afforded to obser\-e, more or less closely, various aquatic animals which abounded on these plants. Their presence in such great numbers suggested the pos- sibility that the Potamogetons might play an important role in the economy of life beyond that of mere shelter and support, or other mechanical and indirect relations which have been ascribed to the larger aquatic plants for many years. It has been stated by Pond (1905) that — The larger aquatic plants, as such, are, while living, little used as food by aquatic animals, yet they greatly increase the surface available for the attachment of microscopic plant 'orms, which are eaten by smaller animals, and the latter in turn by the fishes. In the very recent publication by Shelf ord (191 3), bearing on the life relations of aquatic animals, but little importance is attached to the larger aquatic plants beyond the various mechanical and indirect relations that have so long been attributed to them. He says: The smaller aquatic animals are commonly either alga-eaters or predatory. The larger aquatic animals are commonly predatory or scavengers. The rooted vegetation is eaten only to a small extent. Small floating or swimming plants and animals are the basis of tlie food supply of larger animals. We could probably remove all the larger rooted plants and substitute something else of the same form and texture without greatly affecting the conditions of life in the water; that is, so far as the life habits of the animals are concerned. * * * Plants in water are of particular use to animals as clinging and nesting places. Recent research bids fair to modify these generalizations by Shelford. Such a relation as Pond describes has frequently been observed in P. pectinalus in the autumn when myriads of midge (chironomid) cases have been found applied to the leaves (fig. 56). The leaves are not eaten but they are thickly covered with diatoms and other small algse which, doubtless, afford foraging materials for the lan-as. A small caddis fly (hydroptilid) larva, with characteristic elliptical case, has also been observed in con- siderable numbers in the same relation with peciinatus, the larvae apparently feeding on the epiphytic algal growth. The larvae of both of these insects, after wintering on the algal-covered leaves, have emerged as adults in the spring. Other midges and caddis flies, flies (aquatic Diptera), moths (aquatic Lepidoptera), and beetles (Coleoptera) have been found in great numbers on the various species of Potamogeton. The other smaller invertebrate animals most frequently seen on these plants are Crustacea, snails, and worms. Another interesting relation existing between the Potamogetons and aquatic insect forms is seen in the striking resemblance between the cases of a caddis fly (Leptoceridae) and the stipules of the leaf of P. americanus (fig. 75). The cases in which both larvae and pupae dwell are attached along the stems and leaves in so characteristic a manner as to become almost, if not quite, indistinguishable from the plant parts. 282 BULLETIN OF THE BUREAU OF FISHERIES. Reighard (1894) has expressed in a table "a part of the imperfectly known relation- ships existing between the various groups of plants and the invertebrate animals on the one hand and the fishes on the other." One of the great gaps in the chain of relations therein expressed is a lack of definite knowledge concerning the role of the higher plants. Some definite research in this direction has been begun. Recent investigations on the food habits of aquatic insects have shown that the larger aquatic plants do serve as forage materials. According to Hart (1895), the lar\-ae of NymphiUa sp. (Para- ponyx), an aquatic lepidopterous insect, feed voraciously on Polamogeton natans. Need- ham (1907) mentions the presence of Nym plica advena in the diet of Chironomus alhis- trill, and Morgan (191 2) found that the higher plant tissues formed an important part of the stomach content of May-lly larvae. In view of these investigations the leaves and other edible parts of Potamogeton were closely scrutinized for evidences of their use as food. In my own investigations the first indication that the living tissues of Potamogeton was being eaten was seen in the young growing tips of P. crispus, which had been transferred from a pond to an aquarium in the laboratory. The leaves of several plants were mined by a small larval form which proved to be a chironomid (midge). The characteristic leaf mine is shown in figure 72. Miss Tilbury (1913), who was working in the Cornell laboratory on the feeding habits of the midge, taking advantage of this obsen-ation, reared her species, Chironomus cayugce Johannsen, mainly on P. crispus and entirely on Potamogeton. On examining the leaves of other Potamogetons it was found that practically all species were foraged upon to a greater or less extent. Larval depredations were most common on P. Robbinsii. In this plant the aquatic lepidopterous lar\-a Nymphula sp. (Paraponyx) is the chief herbivore, and so voracious is its appetite that a large proportion of the growing tips are constantly being defoliated in the manner shown by figure 68. Portions of the leaf are cut out also by the larva, applied together by means of silk, and used as a protective case or retreat during the lar^-al and pupal stages. Nymphula sp. is by far the most conspicuous larva feeding upon P. Robbinsii, yet other important smaller forms are numerous. The limy incrustation that accumulates very freely on P. Robbinsii offers apparently especial inducements to certain case-making insects, as midges and caddis flies. Such larvae are exceedingly numerous on this species of plant, and the limy incrustation is the chief material used in the construction of the cases. A few of the chironomid larvae that were common on P. Robbinsii collected at North Fairhaven in October were segregated and fed exclusively on this Potamogeton. They passed successfully through the pupal and adult stages and proved to be the midge, Chironomous aberrans. The larval and pupal stages have been hitherto unrecognized in the life history of this species." The leaves of P. amplijolius were conspicuously mined by the dipterous larva Hydrdlia sp. (Ephydridae). The pupae were collected on the leaves August 6. Several flies and their parasites were reared from them, emergence occurring between August 16 and 20. The larva makes a wide, irregular mine through the leaf, and in each case under observation pupates at the end of the mine toward the base of the leaf blade where the edges naturally roll together and form a protecting furrow (fig. 73). Nymphula sp. (Paraponyx) is also common on this Potamogeton and many of the young leaves are eaten bv them. Oftentimes the lar\a cuts out a portion of the leaf for its case with the neat- o Detcnmnations of dipterous larviE have been made by Prot. O. A. Johannsen: ol caddi».fly lan-ae. by Mr. }. T. Lloyd. POTAMOGETONS IN RELATION TO POND CULTURE. 283 ness and precision of a leaf cutter bee (fig. 74), though usually there is less regularity of outline. On the floating leaves of P. americanns collected early in August were found eggs of Paraponyx and of chironomid." Those of Paraponyx covered broad areas of the under surfaces of the leaves and presented the appearance of minute six-sided cells of honeycomb, yellowish in color. In a few days the larvae hatched and began at once to feed and to cut portions from the leaves for larval cases. Fryer (18S8), in connection with his studies on P. fluitans, mentions that the larvse of Nymphula {H ydrocampa potamogata) entirely destroy the floating leaves of this species, and thus indirectly induce the development of fascicles of leaves, structures which are analogous to the winter buds of P. ohtusijolius. The eggs of the chironomid, which were found on the leaves of P. aviericaniis ,v,e^r& inclosed in small elongate cases blackish in color, suspended from the edges and from the underside of the leaf, and from the petiole. These eggs hatched within a few days, but their entire life history was not observed. The leaves of P. obtusifolius harbor a large number of chironomids, and apparently ofifer a valuable supply of food to many of them. A few of the larv'se were segregated in small dishes and supplied with fresh leaves of this Potamogeton. An undescribed species of Chironomus was reared. Cricotopus trijasciatus and Tanypus flavellus were the most abundant species on the leaves. Other plant parts besides leaves were eaten. The tubers of P. pectinatus and the burs of P. cris pus were devoured by the larvae of Paraponyx and by the larvae of the Chironomidse. The underground stems of P. pectinatus " are provided with large and numerous air spaces (fig. 66), and these were found to be an important air-supplying source for the Donacia larvae. The larvae attached to the subterranean stems of this Potamogeton were collected from the muddy substratum at North Fairhaven August 14, 1913. Stems on which the larvae were not attached showed, quite generally, the characteristic punctures, or double scars, made by the caudal spines in tapping the air supply. Jepson (1905) called attention to the value of the tubers of P. pectinatus in the diet of our wild game birds. He says, "The diving ducks, such as the canvasback and broadbill, eagerly seek these tubers, devoting most of their time to this pursuit until the duck-shooting season opens." McAtee (191 1) and Thompson (1913) in their researches on the diet of wild game birds have shown that a large percentage of the food taken is Potamogeton. The stomach content of 5 canvasbacks has come under my observation recently. One duck shot in October had been feeding in rich aquatic meadows where Potamo- getons flourished with Myriophyllum, Elodea, etc. Its stomach was filled exclusively with tubers of P. pectinatus. Four ducks shot at the close of the season in January had apparently exercised a choice in the matter of food. Feeding in an abundant mixed vegetation, they had selected only Potamogeton — P. Fresii and P. pusillus. The parts of the plants available were the winter buds which at this season have settled in the mud at the bottom along with the hibernacula of Myriophyllum, Elodea, and other aquatic plants. Since this observation was recorded Donacia larvse have been found on the underground stems of P. americanus . 284 BULLETIN OF THE BUREAU OF FISHERIES. SPECIES OF POTAMOGETON AND THE ANIMALS FORAGING UPON THEM. To facilitate reference, a list is given of the species of Potamogeton, together with the smaller animals which have appeared to be intimately associated with them. Other forms of animal life were often found upon these plants, but none seemed to be so char- acteristically on their own ground, so to speak, as the forms listed below. Those animals are starred (*) which have been observed feeding on the living plant tissue. List of Potamogetons and Small Animal r ■ ". -sociated with Them." Plant. Animal. P. americanus Insccta.. Diptcra: Chironomidae (undctcnnined). Lepidoptera: Pyralidre— * Nympliula sp. (Parapon>'x). Tricoptcra: Leptoccridffi — Two species. MoUusca. . Ancylus. P. amplifolius Insecta.. Diptcra: Ephydridae— * Hydrellia sp. Chironomidae — * Chironomussp. * Tanytarsus sp. Tricoptera: Undctmiincd. Lepidoptera: Pyralidrc — * Nymphula sp. (Paraponyx). MoUusca.. Ancylus. P. perfoliatus Insecta. . Diptcra: Chironomida; — * Tanytarsus sp. p. crispus Insecta. . Diptera: * Chironomidae (undetermined). Lepidoptera: Pyralidce — * Nymphula sp. (Hydrocampa). P. zosterifolius Insecta.. Diptcra: Chironomidic — ♦ Tanytarsus sp. Lepidoptera: Pyralidae — * N>Tnphu!a sp. (Paraponji). Tricoptcra: Undetermined. P. obtusifolius Insecta.. Diptcra: Chironomida? — ♦ Chironotnous sp. Cricotopus trifasciatus. Tanytarsus flavellus. p. pectinatus Insecta.. Diptera: Chironomidx — Tanytarsus flavellus Cricoptcris sp. Tricoptera: Hydroplilidar (in autumn). Coleoptera: Donaciine — Donacia sp. P. pectinatus Insecta.. Diptcra: (Gieantic foriD) Chironomidre — Chironomous sp. Tanytarsus sp. Crustacea . . . Amphipoda (very abundant). Gammarus. Hyallela. Eucraneonyx. Venncs.. Nais (very abundant in autumn). P. Robbinsii Insecu.. Diptera: Chironomids — ♦ Chironomus aberrans. Tanytarsus sp. Tricoptcra— Undetermined. Lepidoptera: * Nymphula sp. (ParaponiTc). o A PotaraoKcton which came under casual observation only. P. Epihydrus. may be mentioned as an important addition to the plants actually fttraKtd upon by insect larvjc. Several specimens of this species of Potamoceton. collected at Spencer Lake in August, were quite thickly dotted with caddis-fly larvae (Lcptoccridie), which were (ecdiuK upon the fresh green leaves. POTAMOGETONS IN RELATION TO POND CULTURE. 285 The Mollusca — Planorbis, Limnea, and Physa — were common on all of the Pota- niogetons. These observations on the animal life associated with the Potamogetons afford an additional contribution to the biological relations of the Chironomidae, Pyralidae, Leptoceridae, Hydroptilidas, and Ephydridae, groups in which one or more members have been observed in their feeding operations. Of these animals it has already been recorded by Reighard (1894) that the Chironomidae are an important fish food- Scattered reference is made by others of the value of aquatic insect larvae in the diet of fish. The fact that these insects eat the living plant tissue of the Potamogetons adds greatly to the importance of these plants from an economic standpoint. CONCLUSION. In all contributions bearing on the life conditions of the Potamogetons, the promi- nence of these plants in the shoal waters has been recognized, and where special effort has been directed toward the study of their life relations, an economic value has been ascribed to them. The present investigation affords further evidence of the economic value of these plants, and contributes the results of observation and experiment on the cultivation of several species. These results warrant the expenditure of additional thought and effort on what purports to be one of the most important resources of our lakes, ponds, and streams. BIBLIOGRAPHY. Agardh, J. G. 1852. (Some observations on Potamogeton pcctinatus.) Botanische \xThandlungen dcr K. Schwedisclien Akadcraie dcr Wissenschaften im Jahre 1852. Hnlry t^ikin from a review in Flora, 1854, p. 755-757. AscHERSON, p., and Graebner, P. 1907. Potamogetonaceae, Das Pflanzenreich, 4, 11, p. 184, text fig. 221. Leipzig. Bade, E. 1909. Das Siiswasscr- Aquarium, p. 896. Berlin. Baumann, E. 1911. Die Flora dcs Untersees. Stuttgart. Bennett, Arthi'r. 1880. Notes on pondweeds. Journal of Bot;iny, p. 380. Britton, N. L., and Brown, A. 1913. Illustrated flora of the northern United States, Canada, and the British possessions. 3 vols. New York. Chamisso and Schlechtend. 1827. Linnca, vol. 2, p. 157-233. Clos, D. 1856. Mode dc propagation particulier au Potamogeton crispus L. Bulletin de la Soci£t6 Bo- taniquc de France, t. 3, p. 350-352. Constantin, J. 1884. Recherche sur la structure de la tige des plantes aquatiques. Annales des Sciences Natu- relles, 6° s6r., p. 287-331, pi. 14-17. 1885. Observations critiques sur I'epiderme des feuilles dcs v^g^teaux aquatiques. Bulletin dc la Soci6t6 Botanique de France, t. 32, p. 83-92. 1886. Etudes sur les feuilles des plantes aquatiques. Annales dcs Sciences Naturelles, 7° sir., t. 3, p. 94-162, pi. 2-6. Davis, Chas. A. 1907. The flora of Walnut Lake. In: Report of Biological Survey of Michigan. Published by State Board of Geological Survey as part of report for 1907, p. 217-231. pi. to, 3 maps. Dudley, William R. 1886. The Cayuga flora. Bulletin of Cornell University (Science), p. 132. 2 maps. DvcHE, L. L. 1910. Ponds, jKjnd fish, and pond fish culture. Part i, Bulletin no. 1, State Department Fish and Game, Topeka, Kans., p. 36, fig. 4. EsENBECK, Ernst. 1914. Beitriige zur Biologie der Gattungen Potamogeton und Scirpus. Flora, bd. 7, p. 151-212, text fig. 59. Fischer, G. 1907. Die bayerischen Potamogetonen und Zaunichellien. Berichtc dcr Bayerischen botanische Gesellschaft, bd. 11, p. 20-162. Forbes, S. A. i88o. The food of fishes. Bulletin Illinois State Laboratory Natural History, vol. i, no. 3, p. 18-79- 1888. Studies of the food of fresh-water fishes. Ibid., vol. 2, p. 433-474- 1903. Vegetation. (In his: Plankton of the Illinois River.) Ibid., vol. 6, p. 236-252. Fryer, Alfred. 1888. Notes on ixjndweeds. Journal of Botany, vol. 26, p. 273-278. 1900. The Potamogetons of the British Isles. Parts 7, 8, 9, p. 56. pi. 36. London. 286 BULLETIN OF THE BUREAU OF FISHERIES. 287 Gluck. Hugo. 1905-6. Biologische und Morphologische Untersuchungen fiber Wasser und Sumpfgewachse. 2 vols. Jena. Hankinson, Thos. L. 1907. A biological survey of Walnut Lake, Michigan. Report of Biological Survey of State of Michigan, p. 288, pi. 60, 2 maps. Hart, C. A. 1895. Entomology of the Illinois River and adjacent waters. (In his: Lepidoptera, p. 164-183.) Art. 6, Bulletin Illinois State Laboratory Natural Historj', vol. 4, p. 148-284, pi. 15. Kill, E. J. i8g8. Eleocharis melanocarpa, a proliferous plant. Bulletin of the Torrey Botanical Club, vol. 25, no. 7, p. 392-394, pl- I- 1898. Potamogeton Robbinsii. Botanical Gazette, vol. 25, p. 195-196, pl. 15. HOLFERTY, G. M. 1901. 0\'ule and embryo of Potamogeton natans. Botanical Gazette, vol. 31, p. 239-346, pl. ii-iii. Hull, E. D. 1913. The advance of Potamogeton crispus. Rhodora, vol. 15, no. 177, p. 171-172. Irmisch, Thilo. 1851. Uber die Inflorescenzen der deutschen Potameen. Flora, no. 6, p. 81-93, P'- i- 1858. Uber einige Arten aus der Naturlichen Pflanzenfamilie der Potameen, p. 56, pl. 3. Berlin. JEPSON, W. L. 1903. Where ducks dine. Sunset Magazine, February. Kerner, A., and Oliver, F. W. 1895. The natural history of plants, their forms, growth, reproduction, and distribution. 2 vol. New York. Macgillivrav, Ale.x. D. 1903. DonaciiuEE. (In his: Aquatic Chr>'somelidae, a footnote, p. 325.) Bulletin New York State Museum, no. 68, p. 288-327, pl. 21-31. McAtee, W. L. igii. Three important wild-duck foods. Circular no. 81, Bureau Biological Survey, United States Department of Agriculture, p. 1-19, fig. 19. MiCKLE, G. R. 1912. Possibilities of northern Ontario as a breeding ground for ducks (published by L. K. Cam- eron), p. 3-8, text fig. 2. Toronto. 1913. The increase of the food supply for ducks in northern Ontario. Ibid., p. 17, text fig. 3, figs. i-sb. Morgan, Anna Haven. 1913. A contribution to the biology of May flies. Annals of the Entomological Society of America, vol. 6, no. 3, p. 371-413, text fig. 3, pl. 42-54- MoRONG, Thos. 1893. The Naiadacese of North America. Memoirs of the Torrey Botanical Club, vol. 3, no. 2, 1892-93, p. 65, pl. 55. Needham, J. G. 1907. Notes on the aquatic insects of Walnut Lake. In : A biological survey of Walnut Lake, Mich- igan. Report of Biological Survey of State of Michigan, p. 252-271, fig. 19-21, pl. i, i map. Pieters, a. J. 1894. The plants of Lake St. Clair. Bulletin of the Michigan Fish Commission, p. 10, i map. Lansing. igoi. The plants of western Lake Erie with observations on their distribution. Bulletin United States Fish Commission, vol. 21, p. 57-79, text fig. 9, pl. 8. Washington. Pond, R. H. 1903. The biological relations of aquatic plants to the substratum. Report of United States Com- mission of Fish and Fisheries, p. 483-526, fig. 6. Washington. (In press). The larger aquatic vegetation. American Fresh Water Biology, p. 32, fig. 20. 288 BULLETIN OF THE BUREAU OF FISHERIES. Raunkiaer, C. :903. Anatomical Potamogeton studies and Potamogeton fluitans. Botanisk Tiddskrift, vol. 25, no. 3, p. 253-280, text fig. g. Reichbnbach, Ludovicvs. 1845. Icones Florae Germanicae et Helveticae, vol. 7, p. 40, pi. 71. Reigiiard, J. E. 1894. A biological examination of Lake St. Clair. Bulletin Michigan Fish Commission, no. 4, p. 60, text fig. I, pi. 2, I map. Lansing. Sauvageau, M. C. 1889. Contribution a I'^tude du syst^me mfecanique dans la racine dcs plantcs aquatiques Ics Pota- mogeton. Journal de Botanique, 3° annde, no. 4, p. 61-82, fig. 9. 1890. Observations siu- la structure des feuilles des plantcs aquatiques. Ibid.. 4' ann^e, p. 41, 68, 117, 129, 173, 181, 221, 237. 1891. Sur ks fcuiUcs de quelques Monocotyledones aquatiques. Annales des Sciences Natu- rellcs Botanique, 7'' s^r., p. 103-296. 1894. Notes Biologiques sur les Potamogeton. Journal de Botanique, 8° ann^e, p. i, 21, 45, 98, 112, 140, 166, fig. 31. Shelford, Victor E. 1913. Conditions of existence of aquatic animals, p. 58-72. In his: Animal communities in tem- perate America. Chicago, University of Chicago press, 1913. Sherff, Earl E. 1913. Vegetation of Skokie Marsh. Bulletin of the Illinois State Laborator)', vol. 9, art. 11, p. 575-614, pi. 86-97. SCHENCK, H. 1886. Die Biologic der Wassergewachse , pi. 2. Bonn. Entry taken from a review in Botanisches Ccntralblatt, no. 24, p. 355. Thompson, H. D. 1897. Report on Plants. In: The biological examination of Lake Michigan. Appendix :, Bulletin Michigan Fish Commission, no. 6, p. 72-75. Thompson, R. B. 1913. Description of edible plants. In: The increase of the food supply for ducks in nortliern Ontario. Toronto, p. 8-17, text fig. 2, fig. i-sb. Tilbury, Mary Ruth. 1913. Notes on the feeding and rearing of the midge, Chironomus cayugse johannsen. Journal New York Entomological Society, vol. 21, p. 305-30S. TiTcoMB, John W. 1909. Aquatic plants in pond culture. Bureau of Fisheries, doc. no. 643, 31 p., 32 text fig., 2 pi. USPENSKIJ, H. U. 1913. Zur Phylogenie und Okologie der Gattimg Potamogeton. Moscou. Entry taken from a note in Flora, bd. 7, p. 187. Warming, Eugene. 1909. Oecology of plants. An introduction to the study of plant communities, p. 422. Oxford. Wiegand, K. M. 1898. Embryology of Potamogeton. Botanical Gazette, vol. 25, p. 116-117. 1899. Development of microsporangium and microspores in Potamogeton. Ibid., vol. 28, p. 328-359, pi. 24-25. York, Harlan. 1905. The hibemacula of Ohio water plants. Ohio Naturalist, vol. 5, no. 4, p. 3, fig. 3. Columbus. EXPLANATION OF PLATES. All of figures on Plates XXXIV-XXXIX, with the exception of the photo-micro- graphs, are photographs of plants floating in water, in aqnaria, or in specimen jars. Platb XXII. Fig. I. Polamogeton american-us, seed, 15 j times natviral size. Fig. 2. Potamogeton americanus, seedling 5 days old, i'< times natural size. Fig. 3. Polamogeton americanus, seedWn^ 14 days old, i'< times natural size. Fig. 4. Potamogeton americanus, seedling of four months, J^ natural size; A, winter buds. Fig. 5. Potamogeton americanus. gemiinating winter bud, natural size; the last two winter buds belated in development. Aquarium specimen. January 24, 1914. Fig. 6. Potamogeton americanus, rootstock with winter bud A, natural size. September. PU^TE XXIII. Fig. 7. Potamogeton amplifolius. rootstock, K natural size; A, A, A, young shoots which continue to grow slowly through winter. November. Fig. 8. Potamogeton hctcrophyllus, submerged plant, '2 natural size; A, tuberous rootstock; B, B, submerged shoots; a, b, and c, details of leaves. May 4. Fig. g. Potamogeton heterophyllus, rootstock not tuberous, natural size. July 7. Fig. 10. Potamogeton heterophyllus, X.\xhero\i%Tootsi.ock,-naX\MAs\ze. July 7. Fig. II. Potamogeton heterophyllus, tuberous rootstock, natural size; A, A, incipient shoots November 17. Plate XXIV. Fig. 12. Potamogeton heterophyllus, typical habit of land form, i/; times natural size. Terminal portion of rootstock tuberous. July 7. Fig. 13. Potamogeton heterophyllus, terminal portion of rootstock showing tendency to become tuberous, !},{ times natural size. July 7. Fig. 14. Same, more advanced stage, i'^' times natiu-al size. July 7. Fig. 15. Potamogeton heterophyllus, aquarium specimen, natural size; A, tuberous internode placed in aquarium in November; B, B, new shoots and rootstock. Januarj- 26. Plate XXV. Fig. 16. Potamogeton heterophyllus, aquarium specimen, natural size; A. B, C, D, new shoot and tuberous rootstocks in various stages of growth. March 14. Fig. 17. Potamogeton heterophyllus, aquarium specimen, natural size. Fig. 18. Potamogeton perfoliatus, winter shoot showing elongation of intemodes, natural size; A, leathery scale; a, detail of scale. June. Fig. 19. Growing tips of same. Plate XXVI. Fig. 20. Potamogeton crispus, recumbent branch, showing development 0/ r.'^w shoots on old stem, 5-^ natural size. March. Fig. 21. Potawo^^/oB cm/'u.f, spicular bur, i>^ times natural size. Aquarium. July. Fig. 22. Potamogeton crispus, denticulate bur, 3-^ natural size; a, detail of leaf with denticulate base, 2 times natural size; i, line delimiting starch storage. Fig. 23. Potonoge/on cm/>u!, sprouting bur, i^^ times nattiral size. March. Fig. 24. Potamogeton crispus, denticulate biu- with sprout, natural size. Aquarium. July. Fig. 25. Po/a»i03f(oH crw/jju, spicular bur with sprout, 3; natural size. Aquarium. July. 289 Fig. 2/ natural size Fig. 28 Fig. 29 Fig. 30 290 BULLETIN OF THE BUREAU OF FISHERIES. PuATii XXVII. Fig. 26. Potamogelon crispus, cutting showing bur development, }^ natural size; A, immature burs. Aquarium. March 22-April 7. Potamogelon crispus, sprouting bur, showing development of rootstock and erect shoots, Potamogelon crispus, shoot with spicular bur at base, natural size. Potamogelon crispus, similar structure rooting atx)ve tip of bur, natural size. Potamogelon crispus, bur formation at tip of branch, 1^ times natural size. Aquarium. Platk XXVIII. Fig. 31. Potamogelon crispus, subterranean stem, showing bur in axil of scale, natural size. June. Fig. j2. Potamogelon crispus, sprouting bur. 14 natural size. March. Fig. a. Potamogelon zosterifolius, viinur ]>ud, '4 natural size. October. Fig. 34. Potamogelon zoslerif alius, long section of winter bud, i ' 2 times natural size. Fig. 35- Potamogelon zosterif alius, winter bud sprouting, '/j natural size. April. Plate XXIX. Fig. 36. Po/awo^eton ./i/i/orntif, habit sketch. ,' J natural size. July. Fig. 37. Po(amo9c/o»i//i/ormw. detail of tuberous rootstock, i '4 times natural size. Fig. 38. Potamogelon pcctinatus, young plant developing from tuber A, H natural size. May. Fig. 39. Potamogelon pectinalus, series of tubers, slender form of Dudley, X natural size. Fig. 40. Potamogelon pectinatus, tubers, showing details of early growth, iK times natural size. Plate XXX. Fig. 41. Potamogelon pectinatus, tubers of two successive seasons; A, old; B, young; C, erect stem; D, subterranean stem; i}4 times natural size. Fig. 42. Polamogeton pectinatus, terminal portion of subterranean stem, i}4 times natural size. A condition which may be present from June to October. Fig. 43. Potamogelon pectinatus, mature portion of rootstock bearing tuberous runners; A, tuber- bearing runner; natural size. September. Fig. 44. Potamogelon pectinatus, spray showing fruiting spike and tuberous runners. A, B, C, ^ natural size; a, detail of runner; A, A, young green shoots. Plate XXXI. Fig. 45. Potamogelon pectinatus, plant developed in aquarium, suspended in water, ^ natural size; A, old tuber which produced plant; B, young tuber. Fig. 46. Polamogeton pectinalus, sprouting seed, 3 times nattu-al size. Fig. 47. Same, later stage, 3 times natural size. Fig. 48. Potamogelon pectinatus, seedling about 10 days old, outer testa of seed removed, i>;, times natural size; a, inner hard testa, showing characteristic lid-like portion thrust open, i' j times natural size; b, seedling with hard testa of seed removed showing foot-like expansion. Fig. 49. Polamogeton pectinatus, seedling three weeks old, natural size. Plate XXXI I. Fig. 50. Potamogelon pectinatus, gigantic form of Dudley, runner B, from base of erect shoot A, y^ natural size; C, tuber; D, D, D, young green shoots; i, 2, secondary runners. November. Fig. 51. Polamogeton pectinatus, gigantic form of Dudley, tuber devoid of scales, I'j times natiu^l size. Fig. 52. Potamogelon pectinatus, gigantic form of Dudley, portion of a tuberous runner, natiu-al size. Fig. 53. Potamogelon pectinatus, gigantic form of Dudley, growing tip of secondar>- runner, natural size. EXPI^ANATION OF PLATES. 29 1 Plate XXXII I. Fig. 54. Potamogelon />cc.. Plate XXXVIII. Fig. 69. Potamogeton Robbinsii, photo-micrograph of section through old stem, showing arrange- ment of mechanical tissue. Fig. 70. Detail of fig. 69. Fig. 71. Potamogeton crispus, photo-micrograph of section through stem of starch-filled vegetative structure; a, cell with starch grains. November. Plate XXXIX. Fig. 72. Potamogeton crispus, leaves, showing characteristic leaf mining of chironomids. Fig. 73. Potamogeton ampUfolius, leaves, showing characteristic mines of HyJretlia sp.; a, b, pupa cases at end of mines. August. Fig. 74. Potamogeton ampUfolius; a, b, leaves, showing circular pieces cut away by larva of Nym- phula sp. {Paraponyx); i, larva in case; c, dead leaf, showing cases of Chironomus sp. Fig. 75. Potamogeton americanus , spray showing attachment of cases of caddis fly (fam. Leptoce- rida); a, case of caddis fly; b, stipule of leaf. June 31, 1914. Bull. U. ,S. B. F., igr; Platk XXII. Bull. U. S. B. F., 1913. Platic XXIII. 3i:> 10 Bull. U. vS. B. F., 1913. Plate XXIV. Bull. U. S. B. F.. i.,i,v Plate XXV. Bvih. V. S. B. F., 1 91 3. Plate XXVI. Bui,!.. U. vS. B. F., 1913. Platk XX\'ir. Bull. it. S. B. F., 1913. Pl..\TR XX\"III. Bull. V. S. B. F., 19 13. Plate XXIX. Bull. U. S. B. F., 1913. 42 a Platk Bull. U. S. B. F., 1913. Plate XXXI. 46 49 Bull. U. vS. B. F., 1913. Plate XXXII. Bull. U. S. B. F., 1913. Plate XXXIII. Bull. U. S. B. F., 19 13. Plate XXXIV 58 Bull. U. S. B. F., 19 13. Plate XXX 62 Gl Buix. U. S. B. -p., igi:,. Pl.ATIC XXXA'I. G3 G4 G5 Bull. U. S. P.. F., igi:^- Plate XXXVII. 67 GS Bull. U. S. B. F., 191 1;. Plate XXXVIII. GD Bull. U. S. B. F., 1913. Plate XXXIX. vBlt r 72 74 iO I IBRARY OF CONGRESS DDDD^lSba^o